One goal of an optical imaging system design is to capture nearly error-free images. The optical design thus specifically seeks to correct for certain known optical influences, including, for example, aberrating effects of a medium through which images are captured and unwanted reflections (e.g., scattering) within the imaging system.
Compensating for aberrating effects of the medium is often necessary because the medium unacceptably distorts the optical wavefront, leading to degraded images. The Earth's atmosphere is an example of one medium that can create such degraded images. Turbulent water is another example of such a medium. The only medium that does not affect the optical wavefront is a vacuum at zero atmosphere, which is idealized and practically unachievable.
The prior art has devised adaptive optics to overcome certain problems associated with optical distortions induced by the medium. In typical prior art systems incorporating adaptive optics, information about the medium-induced aberrations is first obtained. After the information is acquired, it is then used to modify or “adapt” the optics of the optical imaging system so as to compensate for the aberrations. The ability of the adaptive optics to compensate for the aberrations is thus directly related to obtaining accurate information concerning the aberrations as generated by the medium.
One prior art technique for obtaining information about aberrations induced by the medium requires direct measurement of phase effects of an optical wavefront traveling through the medium at the aperture stop of the optical imaging system. By measuring the phase of the optical wavefront from a point source with, for example, an interferometer, the optical wavefront may be corrected by changing or “adapting” an optical element, such as a deformable mirror in the optical imaging system. Another term often used to describe adaptive optical elements is “wavefront correction,” which implies that the phase errors of the optical wavefront are corrected at the aperture stop. The aberration-induced effects caused by the medium typically change over time. As the properties of the medium vary, therefore, the point spread function (“PSF”) or spatial impulse response of the optical imaging system also varies. Consequently, the adaptive optics must also change with time, and the phase effects of the optical wavefront must again be determined. These requirements lead to a complex process and a highly involved optical imaging system.
Another prior art technique forms an image of a known object to determine the PSF of the optical imaging system. Typically, this known object is a point source such as a guide star (e.g., non-resolvable star) or a satellite in the field of view of the optical imaging system. Since the PSF is affected by aberrations of the medium, as well as by aberrations specific to the optical imaging system, the PSF may be integrated over the exposure time to acquire the impulse response of both the optical imaging system and the medium. The PSF is then used to deconvolve each subsequent image to obtain a final image that is essentially equivalent to an image that would be obtained if no aberrations were induced by the medium. This technique, however, has a significant shortcoming due to the requirement of a reference point; for example, a non-resolvable star is not often available near the object of interest. In another example, if a satellite serves as a reference, the movement of the satellite makes it difficult to synchronize with primary imaging. In more practical situations on earth, such as imaging ground-based objects with a telescope, there are often no isolated or suitable point reference objects.
Other prior art methods obtain information about aberrations in a medium and do not use an image of a non-resolvable point but attempt to extract information concerning the object from a series of images, while the properties of the aberrating medium change over time. These methods, however, produce images with a high level of noise. Furthermore, attempting to remove all time-varying portions of such images in a series, to obtain a good estimate of the imaged object, requires considerable computing power. In addition, errors are induced when the medium changes and images are taken without the benefit of a current aberration-removing calculation.
In the prior art, one method to compensate for unwanted reflections within and from an optical imaging system is to strategically employ a prism within the system. However, introducing the prism into the path of a converging optical wavefront introduces other aberrations. Moreover, the use of a prism within the system only partially compensates for the unwanted reflections and induces thermal and throughput problems.